This application claims priority to Japanese Patent Application No. P2000-307385.
1. Field of the Invention
The present invention relates to laser diodes, and more particularly, relates to laser diodes having an oscillation wavelength of at least 600 nm and manufacturing techniques therefor.
2. Description of the Background
High-output and high-reliability laser diodes for use in an excitation source for an optical amplifier are used frequently in devices for writing an optical or optomagnetic disk and in optical communication systems. These laser diodes are typically required to operate stably in a fundamental mode for an extended period of time. A insulation coating film may be formed on the surface of a semiconductor as the radiation facet of a laser diode. This process may improve the external differential quantum efficiency by obtaining appropriate reflectance and may prevent a reduction in the peak output caused by an increase in threshold carrier density.
Previous examples of this coating film used in lasers include a silicon oxide thin film formed on one side of a cavity as an antireflection film with a layered film consisting of a silicon oxide thin film and a hydrogenated amorphous silicon thin film formed on the other side of the cavity as a high reflection film. Examples of the coating film are reported by, for example, T. Uasa in Applied Physics Letters, vol. 1, 34, pp. 685).
The antireflection coating film conventionally used in many semiconductor lasers is a silicon oxide, silicon nitride or aluminum oxide single-layer film, or a layered film consisting of a silicon oxide film and a silicon nitride film. These films are designed to achieve a predetermined reflectance value. However, in the case of a high-output laser having an oscillation wavelength of at least 600 nm and a radiation density of several MW/cm2, upon continuous wave operation under auto-power control, the facet degradation of the cavity by optical damage is promoted with the passage of operation time, whereby the laser ceases to oscillate.
The degradation of the radiation facet is caused by a non-radiative recombination center due to a surface state or a defect which occurs on the facet at the time of forming the coating film. When oscillation light is absorbed at the non-radiative recombination center, heat is generated and the temperature of the radiation facet rises. The multiplication of the non-radiative recombination center and a reduction in the energy gap of a forbidden band in the vicinity of the facet occur by heat generation. Thereby, light absorption grows and the temperature of the facet rises further. This may cause the facet to melt or become amorphous, resulting in the irreversible destruction of a laser diode.
In at least one embodiment, the present invention preferably provides a laser diode which has a long service life and high reliability and a method for manufacturing the same.
As will be described in detail below, the present invention may be used in a high-output laser comprising a GaAs substrate having a radiation density of at least several MW/cm2 and an oscillation wavelength of at least approximately 600 nm. According to at least one presently preferred embodiment of the invention, there is provided a laser diode which can be easily manufactured and has a long service life and high reliability with a small increase in the operation current after the laser has been in operation for an extended period of time.
The main aspects of the present invention are as follows. According to a first aspect of the present invention, there is provided a laser diode which has an optical cavity comprised of semiconductor crystals formed on top of a semiconductor substrate and an oxide layer that is substantially free from arsenic oxide formed on at least one side of the optical cavity using the matrix of the cavity as the matrix of the oxide layer (i.e., the main material comprising the optical cavity is the material used for the oxide layer). Herein, the term xe2x80x9csubstantially free from arsenic oxidexe2x80x9d is characterized by a preferred state wherein the oxide layer is totally free from arsenic oxide but, because of imperfections in the manufacturing or other process, this term also includes a minute or trace amount of arsenic oxide in the oxide layer.
According to another aspect of the present invention, there is provided a laser diode which has an optical cavity comprised of semiconductor crystals formed on the top of a semiconductor substrate and which has an oxide region that is substantially free from arsenic oxide and that is continuous to the crystals of the matrix of the cavity with respect to crystallography on at least one side of the optical cavity.
According to another aspect of the present invention, there is provided a laser diode which has an optical cavity comprised of compound semiconductor crystals in the III-V family formed on the top of a semiconductor substrate and which has a layer formed by the hydrogenation and oxygenation of the matrix of the cavity on at least one side of the optical cavity.
According to another aspect of the present invention, there is provided a laser diode which has an insulating film made from a different material from the oxide layer which is substantially free from arsenic oxide.
According to another aspect of the present invention, there is provided a laser diode which has an insulating layer made from a different material from a second layer which is substantially free from arsenic oxide, said second layer formed by the hydrogenation and oxygenation of the matrix of the cavity.
The oxide layer that is substantially free from arsenic oxide or the layer formed by the hydrogenation and oxygenation of the matrix of the cavity of the present invention can be formed on both facets of the optical cavity.
According to another aspect of the present invention, there is provided a laser diode, wherein an optical cavity having semiconductor crystals is formed on top of a semiconductor substrate, and at least one side of the optical cavity has a light reflective surface in the interior of the matrix of the cavity.
That is, in the present invention, the matrix of the optical cavity extends and a light reflective surface is formed in the oxide layer that is substantially free from arsenic oxide or a layer formed by the hydrogenation and oxygenation of the matrix of the optical cavity. In the above laser diode, an area having a light reflective surface in the interior of the matrix of the cavity on at least one side of the optical cavity can be formed on both facets of the optical cavity.
The method of manufacturing a laser diode of the present invention will now be described. According to another aspect of the present invention, there is provided a method of manufacturing a laser diode comprising the step of preparing an optical cavity having compound semiconductor crystals in the III-V family formed on top of a substrate and the step of irradiating the radiation surface of the optical cavity with hydrogen and oxygen.
The present invention may be effective when the hydrogen is excited to an atomic, radical, ion or mixed state thereof and the oxygen is excited to an atomic, radical, ion, ozone or mixed state thereof. The above hydrogen and the above oxygen may be irradiated alternately or at the same time.
The laser diode of the present invention may be used as a compound laser diode having an oscillation wavelength of 600 nm or more. It may also be used as a compound laser diode having an optical output of 1xc3x97106 W/cm2 or more.